Click the DOMAIN INSTALL button. This will trigger a confirmation dialog.

Permission to install.

Click the CONTINUE button. This will trigger a verification page.

Verify permissions and accept terms.

Click the checkbox to accept terms and click the Accept button. The add-on should now be installed
for all accounts in your domain. Keep in mind that it may take up to 24 hours for Google to add the
add-on to all accounts in your domain.

Open any email within Gmail to see the add-on. The add-on is only displayed when looking at a specific email.
It is not shown when looking at a folder of emails (ie. the Inbox or Sent folders).

Note: If you're using Gmail on the desktop, you might need close the Gmail tab/window and open it again
otherwise the newly installed add-on will not appear.

On the web version of Gmail, add-ons appear on the right side of the screen. Click on the shopping cart icon to open the add-on.

Where to find add-ons on web version of Gmail.

On the mobile version of Gmail, add-ons appear at the bottom of the email so you'll need to scroll all the way
down to the bottom to see it. Click on the shopping cart icon to open the add-on.

Where to find add-ons on mobile version of Gmail.

Complete setup by clicking the ACTIVATE button in the add-on.

Add-on activation step.

A new window will open that asks you to login to your Google account on merchantmail.app.
Click on the name of your Google account to proceed.

Note: If have trouble during the activation step, then try again after opening your browser in incognito mode
(also known as private browsing in some browsers) and logging into Gmail.

All done!

At this point, you should be able to start using the add-on. The image below shows a properly configured add-on.